28 Oct 2014

Just like for married couples, communication is
fundamental for cells. When an embryo is developing, its cells need to tell one
another who and where they are, so every tissue and organ grows in
the right place and at the right time. Our neurons are constantly talking to
each other to control our thoughts, feelings and behaviours. Even single-cell
organisms like bacteria can exchange information to decide, for example, how
many times they should multiply.

But how do cells communicate? Scientists have a
good understanding of the key proteins involved in cell communication, or cell
signalling. Typically, a cell sends out a chemical signal (or electrical, in
the case of neurons) that sticks to a specific receptor protein on the surface
of the neighbouring cells. We then say the receptor is ‘activated’, because it
can trigger a cascade of molecular events that ultimately leads to a cellular
response. For instance, the cell might start moving in a particular direction, or a specific
gene gets translated into protein.

There is, however, quite a lot we still don’t know
about cell signalling. What would happen if we could activate a receptor only at
the tip of a moving cell? Would the cell change the direction of migration? And
what if we could activate a receptor repeatedly, or at different time intervals?
Would the cell responses be different? Questions like these have been bugging
scientists for decades, but they simply lacked the tools to address them.

Now, a research team led by Harald Janovjak at the
Institute of Science and Technology (Austria) has developed a new method to
study the fine temporal and spatial regulation of cell signalling using
proteins activated by light. This work opens the way for the development of
powerful approaches to manipulate cell behaviour in health and disease.

Human cells illuminated in a pattern depicting the letters IST. The cells carry a reporter gene that 'glows' when it is triggered with light-activated receptor tyrosine kinases (Credit: Medical University of Vienna).

The
optogenetics revolution

Scientists have been using engineered light-activated
proteins to manipulate cell activity for about a decade or so, a technique that
has been named ‘optogenetics’. The first light-activated proteins, or photoreceptors,
applied in optogenetics belonged to the microbial opsin family. These opsin
photoreceptors are useful because they can move ions across cell membranes in
response to light, a process similar to what triggers neuron activation. In
these initial studies, channelrhodopsins (a type of opsin photoreceptor) were removed from algae and inserted
into particular neuronal cell types in mice. Upon exposure to light, the
neurons containing these proteins started to fire, and depending on which
neurons were activated in this way, a different behaviour was observed in the
mice; in one study, the mice’s levels of anxiety increased, and in another they
started going round in circles.

The reason why optogenetics has been coined a ‘revolutionary
technique’ (and why it is tipped for a Nobel prize) is that it allows scientists
to control the activity of particular cell types or proteins with an
unprecedented level of precision, both in a temporal and spatial manner. And
this, sure enough, comes very handy for cell signalling research. It is a bit
complicated though, to build optogenetic tools for that purpose.

“The main challenges are the same as for many
engineering problems. For example, you want the signalling receptor to be
completely inactive in the “OFF” condition (no light), and to be as much active
as if the natural chemical signal is added in the “ON” condition (light),” says Janovjak.

This fine level of receptor manipulation is very hard
to achieve with conventional optogenetics tools, so Janovjak and
colleagues decided to build signalling receptors activated by light from
scratch, by taking bits and pieces from several proteins and then sticking them
together.

They focused on cell-surface receptors of the receptor
tyrosine kinase (RTK) family, which sense growth factors and hormones and have
been involved in a variety of cellular processes. When an RTK receptor is
activated by a chemical signal, let’s say a growth factor, it attaches to
another receptor in what is called ‘dimerisation’. It is this contact between
two RTK receptor molecules that triggers the molecular events leading to a cell
response, or in other words, that activates RTK signalling. Janovjak and
colleagues knew this, so they looked in bacteria, fungi and plants for proteins
that dimerise in response to light, and then fused them to an RTK receptor
skeleton. In theory, these engineered RTK receptors should dimerise—and
therefore become activated—upon light exposure.

“We were quite beautifully able to do this. In our
study cancer cells with RTKs under optical control quantitatively respond to
light and the growth factor! This is nothing short of amazing and the basis for
all future work by us and others,” says Janovjak.

Manipulating
cell signalling with light

The team showed that when engineered RTKs are inserted
into several cell types, including cancer cells, they can be efficiently
activated by light and induce the predicted cell response very quickly and
within a tiny spatial range.

Morgan Huse, an expert on cell signalling at the Sloan
Kettering Institute (US) says “This study represents the first time that
homodimerising [light-activated] protein domains have been used to activate RTK
signalling. The results are quite significant.”

These new optogenetic tools will be invaluable for
understanding cell signalling, and could also be adapted to study other
cellular processes. In the future, Janovjak’s team will use these tools to
investigate regeneration.

“Our research will focus on regeneration. In essence,
growth factors are known to be efficacious in disease animal models, including
diabetes and Parkinson’s disease. However, delivery of these growth factors is
a real issue because they can induce side effects like (but not limited to)
cancer, and growth factors often can’t reach the desired cells (for example in
the brain). Maybe optogenetics can help”.